I will say about 9/10 people who report someone report them for "verbal abuse or harassment" resulting from 2+ people flaming the shit out of each other after that one play 5 minutes into the game...
I would also say that about 9/10 games that devolve into someone griefing the game (inting...afk...however you wanna see it) are done more out of spite when people start flaming the person for playing poorly, or just not wanting to play the game with people who treat them like shit.
There are cases of people who are just inting the game for either wintrading, or just outright trolling the game for some unspecified reason. RQ from getting rekt is obviously still a thing. But pretty much 95% of the flame wars boils down to how someone delivers and/or receives their messages.
In particular...I am not a nice person. I will 1000% get just as petty with you as you do me. I am terrible at it, and I know it - if you start the flame war with me, this isn't League anymore, it's fucking war of the plebs, and that enemy mid lane has every right to throw that "gg ez", I'll fire back at petty teammates all day. Why? It's pretty simple...
If I'm having a bad game, you best believe I know it. I don't need you telling me I suck. I don't need you rubbing my nose in it. The only thing that gets you, is a pissed off fucking player. I won't int, but I'm sure as hell not gonna think when I'm pissed. It's actually scientifically impossible to think when pissed - your brain QUITE LITERALLY TURNS OFF. You get your fight or flight options - your mind wants to lock itself into fighting (returning the flame with more flame) or pretty much going AFK from the game. Your mechanics get either stupid aggressive, or stupid defensive, to the point you either died immediately doing nothing, or never entered the fight to begin with. It's psychologically proven - tilted people are shit at video games.
If someone is playing poorly, make constructive suggestions - "yo, get a deep ward if you can so you can catch Shaco coming before he gets close to your lane." "Yo, we scale, play back more and we got this."
From experience, I would actually refrain from saying "dude just sit under tower". That's...somewhat defeatist, and pushes the idea that "the only way you're gonna do shit is by humping a tower all game". That's definitely not what you mean...but tilted people dont think, and it comes off to them as you saying they're failing, and the "fight" people are gonna start fucking diving nexus towers cause they're pissed at having to play defensive before you started telling them to.
And obviously, open flames burn your games. If you're actively losing your shit at someone, you're typing instead of playing the game, you're further demoralizing your team, and you're pretty much making sure nobody's gonna listen to you, even if you do have something important to add. Play the MOBA Game, not the Blame Game.
Thank you for coming to my Ted Talk. :P